The invention relates to cellular radio telephone communication systems, and more particularly, to identifying the source of a digital signal in a cellular system.
The cellular telephone industry is growing exponentially in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outgrowing system capacity. If this trend continues, the effects of rapid growth will reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
Throughout the world, one important change in cellular systems is the transition from analog to digital systems. Equally important is the choice of an effective digital transmission scheme. Channel access is often achieved using frequency division multiple access (FDMA) and time division multiple access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal""s transmission power is concentrated. Interference with adjacent channels is limited by the use of bandpass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by radio channels.
In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal""s energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the portion of the interference from different relative signal strength levels is reduced. However, to support more users, the information has to be transmitted in shorter time slots at a faster bit rate.
With FDMA or TDMA systems, or a hybrid FDMA/TDMA system, it is desirable to avoid the case where two potentially interfering signals occupy the same frequency at the same time. In contrast, code division multiple access (CDMA) allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals appear to be on top of each other.
In principle, the information data stream to be transmitted is first coded or spread using a unique spreading code and then combined with a long PN-sequence or a shorter scrambling-sequence. In the latter case, the scrambling-sequences are planned from cell to cell so that neighboring cells use different scrambling-sequences or scrambling-masks. The information data stream and the PN-sequence or the scrambling sequence can have the same or different bit rates. The information data stream and the PN-sequence or the scrambling-sequence are combined by multiplying the two bit streams together. The bits of the unique spreading code and long PN-sequence are usually referred to as chips.
A plurality of coded information signals are transmitted on radio frequency carrier waves and jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, a corresponding information signal is isolated and decoded.
FIG. 1 illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system. Base station 10 transmits a signal 12 that has a maximum signal strength that is limited so as to reduce interference with other base stations. The maximum signal strength of the base station""s transmission creates a foot print or a region within which mobile stations can easily communicate with base station 10. If base station 10 uses a single omni-directional antenna, the foot print extends in an unlimited direction, i.e. 360 degrees. While each footprint is an irregular shape that overlaps with adjacent foot prints, a foot print is often depicted as a hexagon 18 and is usually referred to as a cell.
In a CDMA system, base station 10 can transmit signals to mobile stations 14 and 15 as a single (composite) signal. The signal directed to mobile station 14 is typically coded with a short code that is orthogonal to a short code that is used to code the signal directed to mobile station 15. These signals are spread with a code that is sometimes referred to as a long code. The sum of the two coded and spread signals is then transmitted by base station 10. When mobile station 14 receives the composite signal, mobile station 14 multiplies the spread signal with the long code and the short code to recreate the signal directed to mobile station 14 and the signal directed to mobile station 15 is suppressed as interference noise. Similarly, mobile station 15 multiplies the spread signal with the long code and the short code assigned to mobile station 15 to recreate the signal directed to mobile station 15 and the signal directed to mobile station 14 is suppressed as interference noise. The interference noise is usually not distracting to the user of the mobile station, but as the number of mobile stations increases, so does the level of interference noise. If the omni-directional antenna at base station 10 is replaced with directional antennas, it is possible to divide cell 18 into smaller sectors and thereby reduce system interference. The use of directional antennas increases the capacity of a cellular system and is usually referred to as sectoring.
FIG. 2a illustrates the use of three directional antennas to divide a cell into three 120xc2x0 sectors. Cell 20 has three sectors 21, 22, and 23. FIG. 2b illustrates the use of six directional antennas to divide a cell into six 60xc2x0 sectors. Cell 30 has six sectors 31, 32, . . . , and 36. If base station 10 uses directional antennas, base station 10 can transmit more than one composite signal. When a base station uses directional antennas, each directional antenna transmits to a smaller number of mobile stations than a single antenna would. As a result, the amount of interference decreases and the base station can support a larger number of mobile stations without exceeding an acceptable level of interference noise.
It is sometimes advantageous to transmit the same signal to the same mobile station via more than one source, that is, to provide diversity reception. Sometimes the best source is the source that provides the best signal to noise ratio. Other times the best source is the source that minimizes the interference experienced by the other mobile stations in the system.
As a mobile station moves away from a source (antenna), the quality of the received signal usually decreases. When the quality of the received signal decreases to the point that another source can provide a better signal or the system determines that it can decrease the amount of interference experienced by other mobile stations in the system, the system should perform a handoff. The base station or base stations can perform what is referred to as a soft handoff. A soft handoff occurs when the original source and the new source transmit substantially the same information to the mobile station at the same time and, subsequently, the original source terminates its transmission. If the mobile station is using a RAKE receiver, the signal from the new source appears as additional multipaths, and the RAKE receiver can process the two signals as a single signal.
In some cases, the original base station continues to serve the mobile station, but handsoff the mobile station to a better directional antenna. In other cases, the original base station hands off the mobile station to a neighboring base station. If the neighboring base station has directional antennas, the base station should not only handoff the mobile station to the new base station, but to the directional antenna that provides the best signal.
The mobile station can assist the system in performing the handoff by (1) measuring the quality of signals received from other sources, and (2) reporting these measurements back to the system. If the mobile station receives a better signal from another source, it is helpful if the mobile station can determine the cell number, and if appropriate, the sector number of the better source. This technique is commonly referred to as mobile assisted handoff (MAHO).
When a mobile station is synchronized with a source that is one of a group of sources that are synchronized (or that use a common time reference), it is relatively easy for the mobile station to determine the source of a signal from one of the other sources in the group. For example, assume that a group of base stations are synchronized (or use a common time reference) and that the base stations use the same long code. If the mobile station is synchronized with one base station in the group, the mobile station is synchronized with all the base stations in the group. If each base station shifts the long code a different pre-determined amount, the mobile station can identify the source of signals from other base stations in the group by measuring the amount that the long code has been shifted.
Assume that a second group of base stations is synchronized with or uses the same time reference as the first group. If the mobile station is synchronized with one base station in the first group, the mobile station is synchronized with all the base stations in the second group. The second group of base stations can use a second long code. The mobile station can store or obtain a list of long codes used by the other groups in the cellular system. If each of the base stations in the second group shifts the second long code a different predetermined amount, the mobile station can identify the source (that is, the cell number) of signals from base stations in the second group by calculating how much the second long code has been shifted. If, however, the first group is not synchronized with the second group, the mobile station can not directly identify how much the long code has been shifted.
Assume now that a base station uses directional antennas and that each directional antenna at the base station is synchronized or uses the same time reference. It will be evident to those skilled in the art that directional antennas can use different long codes, and that directional antennas from different base stations can be grouped together. However, assume that each directional antenna uses the same long code. If the mobile station is synchronized with one of the directional antennas, the mobile station is synchronized with all the directional antennas. If each directional antenna shifts the long code a different pre-determined amount, the mobile station can identify the source (i.e., the cell number and the sector number) of signals from directional antennas at the base station by calculating how much the long code has been shifted.
Assume now that a second base station uses directional antennas and that each directional antenna at the second base station is synchronized or uses the same time reference as the directional antennas at the first base station. If the mobile station is synchronized with any one of the directional antennas at the first base station, the mobile station is synchronized with all the directional antennas at the second base station. The second base station can use a second long code. If each directional antenna at the second base station shifts the second long code a different pre-determined amount, the mobile station can identify the source (i.e., the cell number and the sector number) of signals from directional antennas at the second base station by measuring the amount that the second long code has been shifted. If, however, the directional antennas at the second base station are not synchronized with the directional antennas at the first base station, the mobile station can not directly identify how much the long code has been shifted.
In most cases, the only way that the mobile station can identify the source of unsynchronized signals is to connect to the source of the unsynchronized signal and receive information via a broadcast channel or control channel. To connect to and receive information from the broadcast or control channel is timely and costly. Connecting to and receiving information from the broadcast channel or control channel usually involves additional hardware and software and decreases the battery life of the mobile station. Thus, there is a need for a method for a mobile station to directly identify the source (i.e., the cell number or the cell number and the sector number) of a signal from an unsynchronized source.
These and other drawbacks, problems, and limitations of conventional cellular systems are overcome by a method and apparatus for identifying the source of a signal from an unsynchronized source. According to one aspect of the invention, a receiver has an antenna for receiving two signals. The receiver correlates the first signal with a code to find a first location in time, and correlates the second signal with the same code to find a second location in time. The receiver calculates a time difference between the first location in time and the second location in time and uses the time difference to identify the source of the two signals.
According to another aspect of the invention, a cellular system has multiple sources and each source multiplies a signal by a long code before transmitting the signal. Each source shifts the long code by a different amount so that for any two sources there is a unique difference between the shifts in the long code. For example, a first source shifts the long code an amount equal to one; a second source shifts the long code an amount equal to two; and, a third source shifts the long code an amount equal to four. The numbers one, two, and four can correspond to a number of chips or a number of symbols.
Using this example, the difference between the long code shift at the first source and the second source is equal to one, the difference between the long code shift at the second source and the third source is equal to two, and the difference between the long code shift at the first source and the third source is equal to three. If a receiver receives two signals and the difference between the two long code shifts is one, the receiver knows that the two signals are from the first source and the second source; if the difference is two, the two signals are from the second source and third source; and, if the difference is three, the two signals are from the first source and the third source.
According to another aspect of the invention, each signal has a synchronization code that is shifted an amount equal to the shift in the long code. The synchronization code is usually, but not always, repeated in time. For example, if a frame has multiple slots, the synchronization code can be repeated in each slot or every other slot. The synchronization code can be any type of pilot or pilot code that is relatively easy for the mobile station to detect.
Using the example above, the first source shifts the synchronization code an amount equal to one; the second source shifts the synchronization code an amount equal to two; and, the third source shifts the synchronization code an amount equal to four. Because each synchronization code is shifted an amount equal to the shift in the long code, each synchronization code is multiplied by the same portions of the long code. As a result, it is relatively easy for the receiver to determine the relative slot (or frame) position of each synchronization code. For any two sources, there is a unique difference between the relative positions of the synchronization codes. Therefore, if a receiver receives two signals and the difference between the relative positions is one, the receiver knows that the two signals are from the first source and the second source; if the difference is two, the two signals are from the second source and the third source; and, if the difference is three, the two signals are from the first source and the third source.
An advantage of the invention is that it is possible to identify the source of a signal without having to connect to the source to use a broadcast channel or control channel. Another advantage of the invention is that a receiver can distinguish between different synchronized sources without any prior synchronization.